Within the rapidly developing field of nanoscale science and technology, nanomagnetism is of major scientific interest. Nanomagnets promise to be of key technological importance with applications ranging from electronics, including hard discs, magnetic RAM, Giant Magneto Resistance (GMR) devices, and spin valves, through magnetic fluids for industrial uses, and up to biotechnology applications including enhanced imaging of tissues and organs, virus-detecting MRI, and cancer therapy.
Besides NMR, common local magnetic imaging methods include scanning Hall probes, scanning Superconducting Quantum Interference Devices (SQUIDs), Magnetic Force Microscopy (MFM), Lorentz microscopy, Bitter decoration, and magneto-optical imaging. Lorentz microscopy [1] and MFM [2] have a high spatial resolution (10 to 100 nm), however their field sensitivity is relatively low (of the order of 10 Gauss). Scanning SQUID microscopy [3] has the highest field sensitivity (1 μG), but it has a rather poor spatial resolution (of several microns).
A scanning SQUID microscope having high spatial resolution typically uses a magnetic sensor such as a SQUID and a fiber probe magnetically coupled between the SQUID sensor and the sample under study. The fiber probe has a sharply defined tip, and the SQUID has a two-dimensional planar geometry, in which the SQUID loop is fabricated on a flat substrate, and then mounted on the microscope's end. This technique is disclosed for example in US 2005/0057248.
In an alternative technique [4], a Josephson junction is mounted on a conventional tip of a STM (Scanning Tunneling Microscope). The STM is used to maintain a constant distance between the sample and the junction measuring the magnetic field.
In this connection, it should be noted that the Josephson junction is an arrangement of two superconductors with a thin insulating barrier in between, in which a superconducting current flows between the superconductors crossing the barrier even when a zero voltage bias is applied onto them. This barrier can be generalized to some weak link [5], being a normal metal proximity layer, an insulating oxide barrier or some geometrical constriction between the two superconductors.
In a single Josephson junction, the time evolution of the phase difference between wavefunctions (Ginzburg-Landau) of the two superconducting electrodes, Δφ, is given by
                    ⅆ        Δφ                    ⅆ        t              =                            2          ⁢          e                ℏ            ⁢      V        ,where V is the voltage drop across the junction.
Introducing the gauge-invariant phase difference,
  γ  =            Δ      ⁢                          ⁢      φ        -                            2          ⁢          π                          Φ          0                    ⁢              ∫                  A          ·                      ⅆ            s                              the superconducting current (“supercurrent”) in an ideal Josephson junction isIS=IC sin γ,where
      Φ    0    =      hc          e      *      and Ic is the critical current, above which superconductivity is lost i.e. the maximal supercurrent that the junction can carry and where e*=2e.
In a double Josephson junction device, (two Josephson junctions connected in parallel), also known as SQUID, the supercurrents passing through the two junctions in a contour are summed, taking into account the difference in phases between the two electrodes, the flux through a loop is
  Φ  =            ∮              A        ·                  ⅆ          s                      =                                        Φ            0                                2            ⁢            π                          ⁢                              ∫            electrodes                    ⁢                      Δφ            ·                          ⅆ              s                                          +                        ∫          junctions                ⁢                  A          ·                      ⅆ            s                              
Since φ must be single-valued, the sum of the gauge-invariant phase differences (from 1 to 2 plus from 2 to 1, both going clockwise) is 2μΦ/Φ0. If the super-currents passing through the two junctions go from one electrode to the other, their difference must satisfy the following condition:
            γ      1        -          γ      2        =                    2        ⁢        πΦ                    Φ        0              ⁢                  (                  mod          ⁢                                          ⁢          2          ⁢          π                )            .      
Therefore, the maximum supercurrent is flux-dependent and is given by:
            I      max        =          2      ⁢                          ⁢              I        C            ⁢                                cos          ⁡                      (                          πΦ                              Φ                0                                      )                                        ;            I      c        =                  I                  c          ⁢                                          ⁢          1                    +              I                  c          ⁢                                          ⁢          2                    
This result can be used to calculate the magnetic flux through a loop by measuring the critical current, which results in resolving the magnetic flux in the SQUID to very accurate values (as accurate as 10−6Φ0).
General Description
There is a need in the art in providing a magnetic sensor device performing direct magnetic field imaging, having a high spatial resolution. The effective spatial resolution of magnetic sensors is determined not only by the size of the sensors but also by their proximity to the sample. As previously discussed, the existing SQUID technology based sensor devices have size limitations, and the alignment and the scanning of a SQUID sensor, located nanometers above the sample surface, is limited.
The present invention solves the above problem by providing a novel sensor device comprising a probe having a conical tip portion which is configured as a sensor having two superconductors separated by a thin non-superconducting layer (such as a Josephson junction based sensor), where the non-superconducting layer is located at the apex portion of said conical tip, thereby defining electron tunneling region(s) at said apex portion. The technique of the present invention enables the sensor device to be very small and to be brought very close to the sample surface.
It should be noted that a tuning-fork feedback mechanism can be used to approach the surface of a sample. In the tuning fork feedback, or shear force feedback, the tip is mounted to a tuning fork, which is then excited at its resonant frequency. The amplitude of this oscillation is strongly dependent on the tip-surface distance, and it can be effectively used as a feedback signal. This technique involves gluing the probe to a tuning fork and measuring the decrease in the tuning fork's resonant amplitude as it gets closer and closer to the surface of a sample.
The size of the sensor device of the present invention is determined by the size of the pulled tip rather than by the lithographic processing, and the tip can be scanned in extremely close proximity to the sample surface using STM or other SPM (Scanning probe microscope) feedback mechanisms. In addition, such a tip-like sensor device provides for obtaining large bandwidth and reduced noise at higher fields, due to reduced flux trapping in the narrow superconducting leads.
In the present invention, a single or a double Josephson junction based sensor is fabricated on the edge (apex) of a submicron conical tip. The advantage of using a tip as a probe, instead of a planar substrate, is to minimize the distance between the sensor and the sample, and to approach the surface of the sample more accurately.
According to another broad aspect of the present invention, there is provided a local magnetic field sensor device for direct magnetic field imaging, comprising a probe having a conical tip portion which is configured as a sensor having two superconductors separated by a thin non-superconducting layer, such that electrons can cross through the insulating barrier, where at least said insulating barrier (i.e. a tunneling region) of the sensor is located at the apex portion of said conical tip.
The Josephson junction based sensor may be configured for determining the field orientation. The conical tip portion may have a maximal outer diameter that does not exceed a few hundreds of nanometers. The conical tip portion may have a maximal outer diameter of about 100 nm-500 nm. The tunneling region may have a lateral dimension of a few nanometers to several tens of nanometers. The tunneling region may have a lateral dimension of about 10 nm.
In some embodiments, the tip portion, configured as the Josephson junction based sensor, has a core made from an electrical insulator and a superconducting film coating on a selected circumferential region of said insulator core, forming a weak link defining the tunneling region at the apex of said tip. The tip portion configured as the Josephson junction based sensor has a core made from quartz material. The Josephson junction based sensor comprises a continuous superconducting film coating on a selected circumferential region of said quartz core, forming a weak link at the apex of said tip. The superconducting film is selected from aluminum niobium, lead, indium, and tin.
The Josephson junction based sensor is configured and operable to enable a sensitivity of less than 1 Gauss/√Hz, a spatial resolution of less than 100 nm, and a large bandwidth of at least ten kHz.
In some embodiments, the Josephson junction based sensor comprises a SQUID (Superconducting Quantum Interference Device) loop, extending along a circumferential region of the conical tip portion, such that the tunneling regions are located at the apex of said tip. It should be noted that a SQUID consists of a superconducting ring biased with a current I. An external magnetic field H=B/μ is applied to the loop, where μ is the permeability of the material. A Josephson junction is incorporated into each of the two arms of the DC SQUID. The Josephson junctions limit the maximum super-current Ic that can flow across the ring to a maximum value given by the sum of the critical currents of the two junctions. The magnetic flux enclosed inside the SQUID ring modulates periodically, with a period of one flux quantum Φ0=h/2e. This modulation, caused by an interference of the superconducting wave functions in the two SQUID arms, forms the basis of the working principle of the DC SQUID. In this case, the sensor is configured and operable to enable a sensitivity of about 50 mGauss/√Hz and a sensitivity of about 1.75-10−4Φ0/√Hz at a temperature of 300 mK.
The sensor may comprise a single electron transistor (SET) probe, thereby enabling simultaneous nanoscale imaging of magnetic field and electrical potentials having a sensitivity of μV.
According to yet another broad aspect of the present invention, there is provided a method of fabricating a Josephson junction based sensor device. The method comprises providing a conically shaped tip-like substrate made of an electrically insulating material, and coating at least a selected circumferential region of said insulator substrate with a superconducting film so as to form a weak link defining a tunneling region at the apex of said tip.
According to yet a further broad aspect of the invention, there is provided a method of fabricating a Josephson junction based sensor device. The method comprises providing a conically shaped tip-like substrate made of an electrically insulating material, coating at least a selected circumferential region of said insulator substrate with a superconducting film so as to define two film portions spaced-apart from one another at the apex of said tip; and providing an insulator spacer between said two film portions, thereby defining at least one tunneling region at the apex of said tip. The method also comprises insulating a selected region within the tip apex, and then coating the remaining part of the conical tip circumference and the upper insulator with a superconducting film. The selected region is insulated within the tip apex by oxidation of said selected region. The evaporation of the continuous superconducting film may be performed at least at two angles of evaporation. Two films may be evaporated on two opposite sides of a pulled quartz tube.
The two Josephson junctions may be formed to make a circular SQUID loop. The method comprises pulling a tube to define two constriction-based weak links in the cross section of said tube.
In some embodiments, a single electron transistor (SET) probe may be incorporated into said tip. The method comprises coating the circumference of the insulating conical tip with a metallic or a superconducting film patterned to define the two tunneling regions connected in series one to another; coating said tip with an insulating film; and forming a Josephson junction based sensor device above said insulating film selected from a single Josephson junction and a SQUID loop. The method further comprises coating said superconducting layer with Au or Pd—Au so as to protect said tip and make it usable for STM approach in which tunneling current is identified between the end of said tip and a sample.
The critical current of a Josephson junction (JJ) is sensitive to a magnetic field, such that its critical current oscillates as a function of a magnetic field in a way analogous to intensity of light on a screen in a single-slit diffraction experiment, which oscillates as a function of displacement.
According to some embodiments of the invention, the conical tip-like sensor device is configured as a single Josephson junction (i.e. single tunneling region) with either planar or vertical field sensitivity. The vertical field sensitivity can be obtained by coating the tip-like electrically insulating cone with a superconducting continuous film, where the Josephson junction is created as a superconductor-insulator-superconductor tunnel junction at the edge (apex) of the tip. The planar field sensitivity can be obtained by coating a part of the insulating tip-cone circumference with a superconducting layer, then providing upper insulator thereon within the tip apex, e.g. by oxidation, and then coating the remaining part of the tip-cone circumference and the upper insulator with a superconducting film.
According to some other embodiments of the invention, the conical tip-like sensor device is configured as a SQUID (two Josephson junctions connected in parallel). This can be implemented by pulling a tube rather than a rod as in the case of a single Josephson junction. The cross section of the tube naturally forms two weak links.
According to another embodiment of the present invention, the tips can incorporate a single electron transistor (SET) probe [6] thus allowing unprecedented simultaneous nanoscale imaging of magnetic fields with mG sensitivity and electrical potentials with μV sensitivity. The SET device is incorporated into the tip by a two or more layer process. The SET may be deposited as a first layer. The junctions for the single-electron transistor can be made with electron-beam lithography and a standard self-aligned double-angle deposition process. Then, the tip is coated with an insulating layer and the SQUID is deposited as a third layer.
One of the advantages of this approach is that it allows sensitivity to the out-of-plane component of the magnetic field, as well as a comparison between the local magnetic and electrochemical information obtained with the SQUID and SET probes.
According to some other embodiments of the invention, the end of the tip may be cooled (e.g. to a temperature of 70K) by Joule-Thomson refrigeration [12]. Highly pressurized gas can cool the apex of the tip while passing through it, as it expands from high pressure to low pressure. This, of course, requires the SQUID, and possibly the entire tube, to be made from a high-temperature superconductor [13], having a critical temperature higher than the cooled temperature (e.g. 70K). By using this method of refrigeration, it is possible to locally cool only the very end of the tip, possibly enabling the use of the device outside of a liquid-helium/nitrogen Dewar, for use for example on living tissues for biological applications.